Figure 10 is a bar chart showing how the corrosion rate of unprotected zinc corrodes in a sheltered or unsheltered condition in different environments (ASM, 2005c). Interestingly, while the most corrosive situation for zinc is an unsheltered exposure in an industrial environment, the sheltered condition is worse than its unsheltered condition for a marine environment. Accordingly, zinc parts in the dome of a propane tank in a marine environment would have the highest corrosion rates.
Figure 10. Corrosion Rates of Zinc Exposed to Different Atmospheric Environments (ASM, 2005c)
Summary of Corrosion Data and Analysis
Table 18 summarizes the expected performance of the various appurtenances when either buried in soil or under the dome under relatively adverse conditions. The results indicate that the buried condition for the appurtenances is generally more corrosive than the atmospheric exposure condition, especially for the aluminum, brass, ductile iron, steel, and zinc parts. Electrochemically-active materials such as aluminum and zinc are particularly susceptible to galvanic corrosion when coupled to brass component while buried in soil. Traditional magnesium sacrificial anodes would be necessary to protect aluminum and particularly zinc components. If the use of magnesium anodes are not cost-effective, then a robust coating would be needed to ensure long service life if the appurtenances are buried in soils. For a composite tank, the anode would need to be connected directly to the buried appurtenances.
Alternative Underground Propane Tank Materials, Phase II—Final Report
September 2009 Battelle and Lincoln Composites